Tooth mesh characterization of spur gears with tooth root crack and surface pit damage

A finite element/contact mechanics (FE/CM) model is used to characterize the tooth mesh interface of spur gear pairs when the teeth have either root cracks or surface pitting damage. Damage changes the load carried by the teeth and its distribution over the tooth surface. This results in altered deformations of the gears that impacts their static transmission errors and tooth mesh stiffnesses. The FE/CM approach is shown to compare well to published results from the literature for a unity-ratio gear pair, demonstrating the ability of the formulation to incorporate damage on the teeth. The impact of damage is numerically investigated using a nonunity-ratio gear pair from a rotorcraft application. The static transmission errors increase and the tooth mesh stiffnesses decrease with increasing size of pits and increasing lengths of cracks. For pits with fixed widths along the gear face, increasing pit height along the profile increases the effects of damage up to a saturation point. Beyond this point, further increases in pit height lead to increases mostly in the duration of the mesh cycle where the static transmission errors and tooth mesh stiffnesses are impacted by damage. The duration of the mesh cycle affected by root crack damage is closely related to the gear pair contact ratio, regardless of the crack length. The distinct differences in the duration of mesh cycle that is impacted by each damage type may permit its detection from static transmission error or tooth mesh stiffness measurements. The calculated tooth mesh parameters from the characterization of damaged gear teeth are used in a lumped-parameter dynamic model of a gear pair. Dynamic transmission error is calculated and shown to closely agree with direct dynamic response calculations from the FE/CM model. The dynamic response is calculated for varying amounts of damage using the lumped-parameter model. Large tooth surface pits and tooth root cracks are shown to generate sufficiently-large transient response that the gear teeth separate and contact loss occurs. The results of this work may permit the detection of different types of damage from gear pair transmission errors.